When I am asked "what time is it", when should I say "It is 1 o'clock" and when I should say "It is 1 AM /PM"? and in addition, could it be that both of them can be said continuously (It is 1 AM / PM o'clock)?
Speaking both PM/AM and o'clock together is odd!
It's 4 PM o'clock - no, no....
Now, it depends on the context. If someone is asking you now, obviously both of you know the part of the day or night. It makes no sense to add PM if the person is asking you in the afternoon. Yes, this is okay if you are teaching someone when to say PM/AM.
So, if the context is clear, you both know the part of the day, calling it 'o'clock' is common.
Hey, what's the time? ~ It's 3 o'clock (obviously, you both know that it is day and not night.
On the other hand, if you are telling a reference as in the past or in future, calling it with PM/AM is common just to clarify which part of the day/night you are talking about.
My train departs at 4 AM tomorrow
Because here, if you say 'o'clock,' the next question will be when? Early morning or in the evening?
However, what all I said is just in a general context. Special cases always do exist!
How to state the time
For stating times like this, you many options (they do not mix together naturally):
It is 1 o'clock (optional set phrase here). Typically the set phrases are
in the morning,
in the afternoon,
noon(only for 12:00pm),
midnight(only for 12am). If you have a set phrase, you can leave out the
o'clockbut do not need to. There are also set phrases like
on the dotor
sharpto emphasize that the event will happen at the exact time stated.
It's 4 in the afternoon (16:00)
It's 3 o'clock (3:00 or 15:00 depending on context)
It's 7 o'clock in the morning (7:00)
We will meet at 12 noon, correct? (The meeting is at 12:00)
I woke up yesterday morning at 2 o'clock. (The speaker woke up yesterday at 2:00)
It is 1(am/pm)
It common to leave out the o'clock or am/pm when there is no ambiguity when speaking. For example,
When's the next match?
It's at 2
Based on the current time the two speakers can infer the am/pm.
It's 2 by itself is uncommon: the o'clock is needed to specify that the current time is 2.
For business documents
Am/pm or 24-hour time are typically used.
For exact hours (1:00, 2:00, 3:00, ...) it is typical to say
1 o'clock when stating the time because as mentioned above,
It's 1 by itself is ambiguous.
For non-exact hours (1:01, 3:30, 4:12) it is typical to say
three thirty, or
four twelve. These sequences are rare outside of telling time, so they work on their own.